1. Field of the Invention
The present invention relates generally to photographic equipment with motor-driven film advancement and, more particularly, to motor control systems for film advancement.
2. Description of the Related Art
Many photographic cameras include a film drive feature that provides automatic advancement of a filmstrip in the camera after an exposure is produced. Automatic advancement places a fresh, unexposed portion of the filmstrip at an exposure gate of the camera, leaving the camera ready to produce the next exposure. In this way, the camera user is freed from manually advancing the film to produce a series of exposure frames. The film drive feature typically is provided by a drive system having an electric motor that is coupled to the filmstrip by a gear train and a control system that controls the motor to move the filmstrip in a film advance direction by a distance approximately equal to a frame spacing interval.
If a filmstrip is to be commercially processed, the exposure frames on the film should be spaced apart at uniform intervals that are compatible with commercial processing equipment. This is important because most commercial processing is automated and high-speed processing machines are frequently used. Such machines carry out developing and printing, for example, assuming that film to be processed has a standard frame spacing format. If the frame spacing does not conform to the conventional standard, then frames can be processed incorrectly and ruined. For example, prints can be cut off or superimposed on each other.
Cameras with an automatic film drive feature must quickly advance the filmstrip after each exposure by a distance equal to the uniform spacing interval. Any system that provides automatic film drive after an exposure must be able to quickly move the filmstrip to bring a fresh frame at the proper location and then completely stop the film. For example, many cameras are capable of producing exposure frames at a rate of four or five frames per second.
Some of the problems that are encountered by automatic film drive systems are backlash in the gear trains of the motor drive systems and variations in motor load and power source conditions. Backlash refers to the free play in a gear train, which can permit the film to migrate from the position it occupied when the motor was initially stopped, resulting in non-uniform spacing between frames. Variations in load and power occur because of variations in battery strength and intermittent loads from systems such as flash units. Such variations can cause inconsistent motor speed, which can result in inconsistent positioning of the filmstrip.
Another problem faced by automatic film advance systems is film overshoot, which typically occurs when exposures are taken in rapid succession and the film is advanced at relatively high velocities. Overshoot refers to the film coming to rest at a position beyond the intended frame location, causing the frame spacing interval to be other than specified. As film velocities increase, such as might occur when the shutter button is quickly and repeatedly pressed, the inertia of the motor and gear train in a camera can permit the film to travel a distance beyond the intended stop position. This additional travel can alter the frame spacing interval and create the problems noted above, such as incorrect processing and cut-off images.
Various systems have been developed to reduce overshoot and backlash problems, accurately position film, and advance film in accordance with conventional spacing standards. Some systems reverse the direction of the film drive motor after the film has been advanced by one frame. Other systems attempt to lock the film in place. For example, U.S. Pat. No. 5,008,692 to Harvey describes a film advance system having a mechanical pawl that drops into a film perforation. The system reduces the film velocity prior to the film reaching the intended stop position.
Mechanical systems that use cams and pawls for indexing are susceptible to wear, and therefore the accuracy of film placement in such systems can change as the system ages and is used. Therefore, electromagnetic and optical-based systems have been used for film positioning. For example, U.S. Pat. No. 4,987,431 to Harvey describes a film advance system that reads magnetically encoded frame indicia from a photographic film and moves the film either forward one frame or back one frame if film slippage occurs between exposures. In other film drive control systems, optical systems detect the passing of film perforations to provide an approximate location of a film exposure frame. An electromagnetic system, however, requires film movement to read the frame indicia and therefore can be susceptible to overshoot errors. Many optical systems simply attempt to halt film advancement when a particular perforation is first detected. Thus, optical systems of this type assume a standard amount of coasting between the detection of the perforation and the stopping of the film. The amount of coasting, however, depends on the film velocity. Therefore, such a scheme also can be susceptible to overshoot errors. Thus, some of the electromagnetic and optical systems do not provide a relatively high degree of film positioning accuracy and can be susceptible to overshoot.
From the discussion above, it should be apparent that there is a need for a film drive control system that is reliable and accurate, is capable of accommodating high film velocities, and is not subject to problems such as backlash and overshoot, and provides uniform, standard frame interval spacing. The present invention satisfies this need.